1. Field of the Invention
The invention relates to thermal quenching or tempering of glass in quenching or tempering installations where the glass is subjected to the action of a gaseous jet.
2. Description of the Prior Art
In general, the invention may relate to either vertical or horizontal quenching or tempering of glass sheets or plates. Current regulations, particularly with respect to automobile window and roof panes, are quite strict so as to ensure an adequate margin of safety to the user. Increasingly effective quenching or tempering techniques are needed to provide a glass having the mechanical characteristics necessary to meet such regulations, in particular Regulation No. 43 which resulted from the U.N. agreement on uniform conditions of government approval and reciprocal recognition of government approval of motor vehicle parts and equipment.
The regulations require that following breakage of the pane, the number of glass fragments contained in a square of 5.times.5 cm at more than 7.5 cm from the point of impact be between 40 and 350, that no fragment have surface area of greater than 3 cm.sup.2 if not generated within a radius of 7.5 cm of the point of impact, and that no fragment of an elongated shape exist the length of which is greater than 7.5 cm.
Customarily, in order to render glass sheets more resistant so as to meet the regulations, the sheets are heated to a temperature approaching the glass softening temperature, and are then rapidly cooled by blowing air onto the two faces of each sheet simultaneously, the air speed being on the order of 200 m/s and the mass flow rate of the air being very high (possibly attaining 1200 Nm.sup.3 per m.sup.2 (Nm.sup.3 being m.sup.3 at standard temperature and pressure) of glass surface per minute.
Various techniques have been tried aimed at improving the effectiveness of this process, namely by improving the rate of cooling of the glass (which favors the mechanical characteristics desired). The cooling generates temperature gradients between the interior center and the surface of the glass, such gradients extending through the entire thickness of the glass sheet, which gradients continue during the time the glass is cooled through its annealing point, whereby permanent compressive stresses are established in the surface layers of the glass sheet, and lateral stresses develop which are compensated in the interior of the mass of the sheet.
One may, for example, increase the mass flow rate of the air without greatly increasing the nozzle pressure, but this is limited by the problem of setting the dimensions of the panes treated and the parameters of the evacuation of the air blown onto the panes, which evacuation must be carried out without perturbing the blowing action. It is necessary to have sufficient free surface in the installation to allow evacuation of the blast air and the heat picked up by it at the glass. In current installations, where the glass-support equipment and the blowing nozzles together occupy a substantial amount of surface, an increase in the flow rate, which is accompanied by an increase in the space claimed by the nozzles and the blast air itself, still further reduces the space available for evacuation, and is detrimental to cooling efficiency and effectiveness.
The technique has also been tried of varying the air jets in time and space (pulsation of the flow, oscillation of the blowing apparatus, etc.), and also that of intensifying the blowing in certain zones of the sheet surface, e.g., alternating bands of strong cooling with bands of less intense cooling. However, this introduction of additional complexity into the means employed has resulted only in a reduction in the number of needles after breakage of the pane, namely the number of fragments over 7.5 m long, and has not resulted in improvement in the other parameters governed by Regulation No. 43.
Finally, given the approximate proportionality between the heat transfer from the glass being cooled to the air and the velocity of that air, the technique has been tried of increasing the supply pressure of the blowing nozzles with the aim of increasing the air velocity. In this connection, pressures have been employed which are associated with sonic or even supersonic velocities. However, this entails a substantial increase in energy consumption, due to the inevitable high pressure drop in particular; and manufacturing costs are increased to a degree inconsistent with industrial feasibility.
Another consideration is the continuing current effort to reduce the weight of vehicles, which logically may be accomplished by employing thinner panes, namely about 3 mm thick or less. In order to ensure the proper mechanical characteristics of such panes, it is necessary to further intensify cooling, and thus to develop more powerful and effective methods of quenching or tempering.